In recent years examination and experimentation at the atomic level has increased as part of the quest for knowledge of the basic building blocks of matter.
For example, many experiments have been developed that use stimulated Raman transitions to examine the behavior of an atom sample. Such stimulated Raman transitions use counterpropagating laser beams to stimulate an atom sample and change its energy levels. However, these stimulated Raman transitions that couple atomic ground states with counterpropagating laser beams are resonant only within a narrow velocity band. This phenomenon, known as atomic velocity selection, has proven to be a useful tool for a variety of experiments, including subrecoil Raman cooling, atom interferometry, and atom velocimetry. See U.S. Pat. No. 5,274,232 to Chu et al.; see also M. Kasevich, et al., Atomic Velocity Selection Using Stimulated Raman Transitions,” Phys. Rev. Lett. 66, 2297 (1991); V. Boyer, et al., “Deeply subrecoil two-dimensional Raman cooling,” Phys. Rev. A 70, 043405 (2004); J. M. McGuirk, “Sensitive absolute-gravity gradiometry using atom interferometry,” Phys. Rev. A 65, 033608 (2002); and J. Chabé, et al., “Improving Raman velocimetry of laser-cooled cesium atoms by spin-polarization,” Opt. Commun. 274, 254 (2007).
Stray magnetic fields can adversely affect this process by shifting the magnetic sublevels, thereby perturbing the participating velocity bands. See M. Kasevich et al., supra, and J. Chabé, et al., supra. See also J. Ringot, et al, “Subrecoil Raman spectroscopy of cold cesium atoms,” Phys. Rev. A 65, 013403 (2001). Measurement of vector magnetic fields with magnetoresistive probes has been used for active compensation of both dc and ac fields, but needs several sensors placed externally to the vacuum chamber. See J. Ringot, et al., supra.
Elimination of stray fields to submilliGauss levels is particularly important for subrecoil cooling processes. See V. Boyer, et al., supra; V. Vuletić, et al., “Degenerate Raman Sideband Cooling of Trapped Cesium Atoms at Very High Atomic Densities,” Phys. Rev. Lett. 81, 5768 (1998). Typically, these fields are nulled by Helmholtz coils along each Cartesian direction. Correct compensation currents can roughly be estimated by visual indicators such as atom expansion in an optical molasses, but these cues are strongly dependent on optical alignment. Stray fields can be directly measured using, for example, Faraday spectroscopy, which provides picoTesla sensitivity, but requires additional laser frequencies and time-resolved polarimetry. See T. Isayama, et al., “Observation of Larmor spin precession of laser-cooled Rb atoms via paramagnetic Faraday rotation,” Phys. Rev. A 59, 4836 (1999); G. A. Smith, et al, “Faraday spectroscopy in an optical lattice: a continuous probe of atom dynamics,” J. Opt. B: Quantum Semiclassical Opt. 5, 323 (2003); and G. Labeyrie, et al., “Large Faraday rotation of resonant light in a cold atomic cloud,” Phys. Rev. A 64, 033402 (2001).
These and other experiments and applications that use ultracold atoms thus require a measure of the magnetic field at the atom sample. Because these atom samples are housed in a vacuum chamber at ultrahigh vacuum, i.e., 10−9 Torr or lower (UHV), optical techniques that interrogate the atom sample with a probe laser beam are required. Mechanical probes cannot access the interior of the chamber without disrupting the integrity of the vacuum.
The optical techniques traditionally used rely on magneto-optic polarization rotation of a probe laser beam. See generally H. J. Metcalf et al., Laser Cooling and Trapping (1999). Techniques based on magneto-optic rotation are sensitive but cumbersome to implement. They require multiple laser beams and frequencies, high quality polarization optics, good timing resolution, balanced photodetection, and good optical alignment.